Chronic graft versus host disease (cGVHD), a systemic autoimmune syndrome, remains the major cause of morbidity and mortality of long-term survivors of allogeneic hematopoietic cell transplantation (HCT). The long-term goals of our project are to dissect the cellular and molecular mechanisms of cGVHD pathogenesis, and to develop effective therapies for prevention and treatment of cGVHD. The proposed studies will dissect the mechanisms whereby graft and de novo- generated CD4+ T cells interact with graft and de novo-generated B cells to induce cGVHD and clarify whether blockade of GC formation can prevent induction of cGVHD. The proposed studies will also test whether interactions among CD4+ T, B, and dendritic cells in target tissues early after disease onset allow cGVHD to persist during lymphopenia. We have recently developed a cGVHD model using MHC-mismatched C57BL/6 donors and BALB/c recipients. Results with this model closely reflect the transition from acute to chronic GVHD and characteristic features observed clinically in patients. Donor CD4+ T cells induced cGVHD in the presence or absence of host thymus. But donor CD8+ T cells induced cGVHD only in recipients with a functioning thymus, in which donor CD8+ T cells damaged thymic negative selection, resulting in generation of autoreactive CD4+ T cells that mediate cGVHD. Donor-type B cells from the graft and de novo generation after HCT augment induction of cGVHD by donor CD4+ T cells. Thus, autoimmunity in cGVHD is triggered by alloimmunity that eliminates host-type follicular dendritic cells, thereby preventing formation of germinal centers (GC). Taken together, these results suggest that GC formation is not necessary for development of cGVHD. On the other hand, results from other investigators have suggested that GC formation is necessary for development of cGVHD. This project is designed to test three related hypotheses relevant to the role of interactions between CD4+ T cells and B cells in the pathogenesis of cGVHD. 1) Mature donor CD4+ T cells from the graft interact with mature graft B cells and de novo-generated B cells after HCT to induce cGVHD in the absence of GC formation, although certain interventions that disrupt interactions between CD4+ T and B cells can prevent cGVHD. 2) Interactions between de novo-generated CD4+ T and B cells may lead to GC formation, but autoimmunity rapidly destroys the GCs and lymphoid tissues, and interventions that block GC formation are not expected to prevent cGVHD. Again, however, disruption of interactions between CD4+ T and B cells can prevent cGVHD. 3) The persistence of cGVHD under lymphopenia requires interactions between CD4+ T cells, B cells, and dendritic cells in cGVHD target tissues. The proposed studies will provide new insights into how CD4+ T and B cells interact to induce and perpetuate cGVHD and will lead to the development of novel regimens for prevention and treatment of cGVHD.